The present specification generally relates to network controlled device-to-device (D2D) communications scenarios. In view of the 3rd Generation Partnership Project (3GPP) mobile network technology Long Term Evolution Advanced (LTE-A) Rel-12, the present specification in particular addresses LTE-A support for network-controlled D2D communications. Network controlled D2D communications include, for example, direct communications in a cluster of proximity devices, and optimized data path via eNB(s) in cellular network.
Since D2D may provide the opportunity to relieve certain network elements from part of traffic load, D2D related topics are under discussion within 3GPP and also are also of high interest of different network operators.
In this specification, in particular D2D communications with optimized data path (known as locally routed data path in 3GPP TR22.803) and direct data path mode are considered. D2D communications with optimized data path can be seen as communications between two terminals (e.g. user equipment (UE)) via the respective serving access nodes (e.g. evolved Node B (eNB)) without routing the data path via gateways (e.g. serving gateway (S-GW), packet data network gateway (P-GW)) of the respective operator network. D2D communications with direct data path can be seen as communications between two terminals directly, i.e. without routing the data path via the respective serving access nodes and/or gateways of the respective operator network. It is noted that the above description only mentions the data path, while the control path may nevertheless involve the respective serving access nodes and/or gateways of the respective operator network. In 3GPP TR22.803, a detailed description of different D2D communication modes in relation to data path and control path is given.
For D2D communication with optimized data path and direct data path mode in a network controlled way, i.e., involving the respective serving access nodes and/or gateways of the respective operator network for the control path, mobility procedure could happen to either or both of the D2D pair of UEs (a pair of UEs utilizing D2D communication with each other). For example, one of the D2D pair (or both of the D2D UEs) is/are moving into the coverage of an adjacent eNB, and a radio handover of one of the D2D pair (or both of the D2D UEs) occurs. According to 3GPP TR22.803, in such case the D2D service continuity is to be guaranteed as much as possible to provide good user experience. During the radio handover procedure, if conventional evolved packed system (EPS) service is also ongoing, the radio handover procedure involves mobility management entity (MME)/core network (CN) for path switching, security key updating, and so on.
However in case no EPS service is associated with the D2D pair, it may be possible that this D2D pair is in RRC_Connected state while MME/CN is in ECM_IDLE state (which means that RRC_Connected state is maintained for D2D service only). Hence when mobility happens to the D2D pair (i.e. one or both UEs are handed over), the core network node, for example MME/D2D registration server function (DRSF) is sometimes not immediately needed to be involved for radio handover procedures.
In sum, for a D2D service via optimized path and direct path between two UEs it may be possible that only D2D service is ongoing but no EPS service is active. In this case the D2D pair of UEs is in RRC_Connected state and seen in ECM_IDLE state seen by MME/CN.
Hence, the problem arises that in view of lack of necessity of involvement of the CN in radio handover procedures in certain situations, such involvement is to be minimized as much as possible during the D2D pair mobility. In particular, radio handover procedures in case of mobility of D2D communicating UEs at radio access network (RAN) level are to be defined. More specifically, radio handover procedures for optimized path and direct path D2D UE are to be defined.
Hence, there is a need to provide for mobility management for device-to-device communications.